1. Field of the Invention
This invention relates to a radio system having a fairly large number of transmitting and receiving stations (exchanges) spatially distributed over an area to be covered, and more particularly to such a system wherein each exchange preferably serves a fairly large number of individual subscribers in such a way that the individual subscriber can reach a subscriber who may be connected to another exchange, in particular by way of a connection which is established between the individual subscribers according to German Pat. No. 1,299,734.
2. Description of the Prior Art
It is essential for systems of the type described above to contain a fairly large number of exchanges with are provided in a network-like distribution over the area to be served. Each subscriber connected to any exchange of the network can therefore reach any other subscriber connected to an exchange of the network which is not yet interfered with a high safety factor, due to a particular connection construction, and possibly while utilizing the exchange purely as a transit or relay station. The individual exchanges of the system are thereby primarily interconnected by way of radio paths. The preparation of the individual speech channels is mostly provided in such a way that they are combined according to a time grouping method to become a modulation signal for the radio frequency carrier. The modulation signal then serves for the phase or frequency modulation of the radio frequency carrier. Often, the signals of the individual speech channels are previously converted into a pulse code modulation signal or into a pulse delta modulation signal for grouping of the speech channels. This entails the advantage that the essential noise voltage within the individual speech channels are merely the quantizing noises. Mainly when such an information transmission system operates with a large number of radio transmission paths, the construction expense of the apparatus within the individual exchange is relatively great. It should be pointed out that a complete directional radio device will be required for each connection to an adjacent exchange of the network. Therefore, it must be taken into account that the application of directional radio connections limits the guidance of traffic of the information between adjacent exchanges, since the conventional directional radio devices are designed for a certain number of speech channels. This is due to the fact that an essentially higher number of speech circuits must be provided for the individual directional radio connection in order to avoid overloading of the exchange during relay traffic than would ordinarily be found with respect to normal traffic.
The present invention is primarily concerned with providing not only a lower construction expense within the exchanges of a radio system of the type described above, including provision for the so called relay traffic, but also to provide an improvement in the flexibility of directing information within the radio system.
The foregoing object is achieved in a radio system of the type initially mentioned, according to the invention, in such a way that the radio frequency channels with different frequencies positioned in a frequency range determined for the system are provided in the radio system at least for the speech channels of adjacent exchanges. The transmission of the individual speech signal (speech channel) is provided according to a digital method, preferably by converting the signals of the individual speech channels into pulse code modulated (PCM) signals or into pulse delta modulation signals. Also the bandwidth is preferably reduced for the individual signals through the use of a vocoder device. Within an individual exchange the transmitter transmits its information toward the exchanges which are to be connected by way of an antenna which, at most, will group little in the plane spatially determined by the exchanges, and is preferably embodied as an omnidirectional antenna. Directional antennas, preferably having a relatively high grouping capability in the horizontal plane and in the vertical plane are provided for receiving the radio frequency signals. The directional antennas are directed toward the other exchanges so that all exchanges which can be reached by the transmitter of an individual exchange are reached by way of the omnidirectional antennas, while the individual receivers in each exchange respectively receive from the transmitter of another exchange by way of an associated directional antenna.
It is advantageous when several respectively adjacent frequency radio frequency speech channels form a speech channel group which is treated as a unit in the radio frequency range and from which the speech channels must respectively be selected for connection are obtained and exchanged in the intermediate frequency range of the individual receiver.
It is furthermore advantageous when the associated receiver for each direction of reception only includes the radio frequency part of a superheterodyne receiver and has an intermediate frequency output, and that the speech channels which must respectively be exchanged are selected on the basis of frequency from the intermediate frequency signals of the individual receivers in the same frequency position and are placed into the form of the original digital signal or even the original analog signal by means of demodulation. The signals of the channels are processed in this form either to the subscribers connected to this particular exchange or, in the case of relay traffic, are extended toward the transmitter of the exchange after a corresponding preparation, preferably while analogously reconverting the signals at an intermediate frequency.
It is furthermore advantageous to provide a regenerating circuit for the signals provided in the digital form in the demodulation path and/or in the case of relay traffic, in the preparation path for the intermediate frequency signal which must be further processed.